Panel-form loudspeakers

ABSTRACT

A panel-form loudspeaker has two distributed mode acoustic radiators coupled by a resilient suspension. Each radiator has its own transducer.

This application is a continuation-in-part of application Ser. No.08/707,012, filed Sept. 3, 1996.

TECHNICAL FIELD

The invention relates to loudspeakers and more particularly toloudspeakers comprising panel-form acoustic radiating elements.

BACKGROUND ART

It is known from GB-A-2262861 to suggest a panel-form loudspeakercomprising:

a resonant multi-mode radiator element being a unitary sandwich panelformed of two skins of material with a spacing core of transversecellular construction, wherein the panel is such as to have ratio ofbending stiffness (B), in all orientations, to the cube power of panelmass per unit surface area (μ) of at least 10;

a mounting means which supports the panel or attaches to it a supportingbody, in a free undamped manner;

and an electro-mechanical drive means coupled to the panel which servesto excite a multi-modal resonance in the radiator panel in response toan electrical input within a working frequency band for the loudspeaker.

U.S. Pat. No. 3,247,925 of WARNAKA discloses what purports to be a lowfrequency resonant panel loudspeaker mounted in a chassis and excited byan electromechanical transducer mounted on the chassis.

DISCLOSURE OF INVENTION

Embodiments of the present invention use members of nature, structureand configuration achievable generally and/or specifically byimplementing teachings of our co-pending application Ser. No.08/707,012. Such members thus have capability to sustain and propagateinput vibrational energy by bending waves in operative area(s) extendingtransversely of thickness often but not necessarily to edges of themember(s); are configured with or without anisotropy of bendingstiffness to have resonant mode vibration components distributed oversaid area(s) beneficially for acoustic coupling with ambient air; andhave predetermined preferential locations or sites within said area fortransducer means, particularly operationally active or moving part(s)thereof effective in relation to acoustic vibrational activity in saidarea(s) and signals, usually electrical, corresponding to acousticcontent of such vibrational activity. Uses are envisaged in co-pendingapplication Ser. No. 08/707,012 for such members as or in "passive"acoustic devices without transducer means, such as for reverberation orfor acoustic filtering or for acoustically "voicing" a space or room;and as or in "active" acoustic devices with transducer means, such as ina remarkably wide range of sources of sound or loudspeakers whensupplied with input signals to be converted to said sound, or in such asmicrophones when exposed to sound to be converted into other signals.

This invention is particularly concerned with active acoustic devices inthe form of loudspeakers. Members as above are herein called distributedmode acoustic radiators and are intended to be characterised as in theabove co-pending parent application and/or otherwise as specificallyprovided herein.

The invention is a panel-form loudspeaker having a member comprising astiff lightweight panel having capability to sustain and propagate inputvibrational energy by bending waves in at least one operative areaextending transversely of thickness to have resonant mode vibrationcomponents distributed over said at least one area and havepredetermined preferential locations or sites within said area fortransducer means and having first and second transducers mounted on saidmember at two of said locations or sites to vibrate the member to causeit to resonate forming an acoustic radiator which provides an acousticoutput when resonating. The first and second, transducers may be adaptedto operate in different frequency ranges. The radiator may have acellular core sandwiched between skins. The loudspeaker may comprise aframe supporting the radiator, and a resilient suspension by which theradiator is attached to the frame. The frame may surround the radiator,and the suspension may be attached to the edge of the radiator. Thefirst and second transducers may be mounted wholly and exclusively onthe radiator. One of the transducers may be electromagnetic. One of thetransducers may be piezo-electric. The panel-form loudspeaker maycomprise a second member having capability to sustain and propagateinput vibrational energy by bending waves in at least one operative areaextending transversely of thickness to have resonant mode vibrationcomponents distributed over said at least one area and havepredetermined preferential locations or sites within said area fortransducer means and having a transducer mounted on said member at oneof said locations or sites to vibrate the member to cause it to resonateforming an acoustic radiator which provides an acoustic output whenresonating, the second member being mounted on or in the first saidmember, and a resilient suspension coupling the first and secondmembers. The second member may be mounted in an aperture in the firstmember. The second transducer may be mounted wholly and exclusively onthe second member.

BRIEF DESCRIPTION OF DRAWINGS

The invention is diagrammatically illustrated, by way of example, in theaccompanying drawings, in which:

FIG. 1 is a diagram showing a distributed-mode loudspeaker as describedand claimed in our co-pending application No. 08/707,012;

FIG. 2a is a partial section on the line A--A of FIG. 1;

FIG. 2b is an enlarged cross-section through a distributed mode radiatorof the kind shown in FIG. 2a and showing two alternative constructions;

FIG. 3 is a diagram of a first embodiment of distributed-modeloudspeaker according to the present invention;

FIG. 4 is a diagram of a second embodiment of distributed modeloudspeaker according to the invention;

FIG. 5 is a diagram of a third embodiment of distributed modeloudspeaker according to the invention;

FIG. 6 is a diagram of a fourth embodiment of distributed modeloudspeaker according to the invention, and

FIG. 7 is a perspective diagram of a transducer.

BEST MODES FOR CARRYING OUT THE INVENTION

Referring to FIG. 1 of the drawings, there is shown a panel-formloudspeaker (81) of the kind described and claimed in our co-pendingapplication No. 08/707,012 comprising a rectangular frame (1) carrying aresilient suspension (3) round its inner periphery which supports adistributed mode sound radiating panel (2). A transducer (9) e.g asdescribed in detail with reference to our co-pending application Ser.Nos. 09/011,773, 09/011,770 and 09/011,831 is mounted wholly andexclusively on or in the panel (2) at a predetermined location definedby dimensions x and y, the position of which location is calculated asdescribed in our co-pending application Ser. No. 08/707,012, to launchbending waves into the panel to cause the panel to resonate to radiatean acoustic output.

The transducer (9) is driven by a signal amplifier (10), e.g. an audioamplifier, connected to the transducer by conductors (28). Amplifierloading and power requirements can be entirely normal, similar toconventional cone type speakers, sensitivity being of the order of 86-88dB/watt under room loaded conditions. Amplifier load impedance islargely resistive at 6 ohms, power handling 20-80 watts. Where the panelcore and/or skins are of metal, they may be made to act as a heat sinkfor the transducer to remove heat from the motor coil of the transducerand thus improve power handling.

FIGS. 2a and 2b are partial typical cross-sections through theloudspeaker (81) of FIG. 1. FIG. 2a shows that the frame (1), surround(3) and panel (2) are connected together by respective adhesive-bondedjoints (20). Suitable materials for the frame include lightweightframing, e.g. picture framing of extruded metal e.g. aluminium alloy orplastics. Suitable surround materials include resilient materials suchas foam rubber and foam plastics. Suitable adhesives for the joints (20)include epoxy, acrylic and cyano-acrylate etc. adhesives.

FIG. 2b illustrates, to an enlarged scale, that the panel (2) is a rigidlightweight panel having a core (22) e.g. of a rigid plastics foam (97)e.g. cross linked polyvinylchloride or a cellular matrix (98) i.e. ahoneycomb matrix of metal foil, plastics or the like, with the cellsextending transversely to the plane of the panel, and enclosed byopposed skins (21) e.g. of paper, card, plastics or metal foil or sheet.Where the skins are of plastics, they may be reinforced with fibres e.g.of carbon, glass, Kevlar (RTM) or the like in a manner known per se toincrease their modulus.

Envisaged skin layer materials and reinforcements thus include carbon,glass, Kevlar (RTM), Nomex (RTM) i.e. aramid etc. fibres in various laysand weaves, as well as paper, bonded paper laminates, melamine, andvarious synthetic plastics films of high modulus, such as Mylar (RTM),Kaptan (RTM), polycarbonate, phenolic, polyester or related plastics,and fibre reinforced plastics, etc. and metal sheet or foil.Investigation of the Vectra grade of liquid crystal polymerthermoplastics shows that they may be useful for the injection mouldingof ultra thin skins or shells of smaller size, say up to around 30 cmdiameter. This material self forms an orientated crystal structure inthe direction of injection, a preferred orientation for the goodpropagation of treble energy from the driving point to the panelperimeter.

Additional such moulding for this and other thermoplastics allows forthe mould tooling to carry location and registration features such asgrooves or rings for the accurate location of transducer parts e.g. themotor coil, and the magnet suspension. Additionally with some weakercore materials it is calculated that it would be advantageous toincrease the skin thickness locally e.g. in an area or annulus up to150% of the transducer diameter, to reinforce that area and beneficiallycouple vibration energy into the panel. High frequency response will beimproved with the softer foam materials by this means.

Envisaged core layer materials include fabricated honeycombs orcorrugations of aluminium alloy sheet or foil, or Kevlar (RTM), Nomex(RTM), plain or bonded papers, and various synthetic plastics films, aswell as expanded or foamed plastics or pulp materials, even aerogelmetals if of suitably low density. Some suitable core layer materialseffectively exhibit usable self-skinning in their manufacture and/orotherwise have enough inherent stiffness for use without laminationbetween skin layers. A high performance cellular core material is knownunder the trade name `Rohacell` which may be suitable as a radiatorpanel and which is without skins. In practical terms, the aim is for anoverall lightness and stiffness suited to a particular purpose,specifically including optimising contributions from core and skinlayers and transitions between them.

Several of the preferred formulations for the panel employ metal andmetal alloy skins, or alternatively a carbon fibre reinforcement. Bothof these, and also designs with an alloy Aerogel or metal honeycombcore, will have substantial radio frequency screening properties whichshould be important in several EMC applications. Conventional panel orcone type speakers have no inherent EMC screening capability.

In addition the preferred form of piezo and electro dynamic transducershave negligible electromagnetic radiation or stray magnetic fields.Conventional speakers have a large magnetic field, up to 1 meter distantunless specific compensation counter measures are taken.

Where it is important to maintain the screening in an application,electrical connection can be made to the conductive parts of anappropriate DML panel or an electrically conductive foam or similarinterface may be used for the edge mounting.

The suspension (3) may damp the edges of the panel (2) to preventexcessive edge movement of the panel. Additionally or alternatively,further damping may be applied, e.g. as patches, bonded to the panel inselected positions to damp excessive movement to distribute resonanceequally over the panel. The patches may be of bitumen-based material, ascommonly used in conventional loudspeaker enclosures or may be of aresilient or rigid polymeric sheet material. Some materials, notablypaper and card, and some cores may be self-damping. Where desired, thedamping may be increased in the construction of the panels by employingresiliently setting, rather than rigid setting adhesives.

Effective said selective damping includes specific application to thepanel including its sheet material of means permanently associatedtherewith. Edges and corners can be particularly significant fordominant and less dispersed low frequency vibration modes of panelshereof. Edge-wise fixing of damping means can usefully lead to a panelwith its said sheet material fully framed, though their corners canoften be relatively free, say for desired extension to lower frequencyoperation. Attachment can be by adhesive or self-adhesive materials.Other forms of useful damping, particularly in terms of more subtleeffects and/or mid- and higher frequencies can be by way of suitablemass or masses affixed to the sheet material at predetermined effectivemedial localised positions of said area.

An acoustic panel as described above is bidirectional. The sound energyfrom the back is not strongly phase related to that from the front.Consequently there is the benefit of overall summation of acoustic powerin the room, sound energy of uniform frequency distribution, reducedreflective and standing wave effects and with the advantage of superiorreproduction of the natural space and ambience in the reproduced soundrecordings.

While the radiation from the acoustic panel is largely non-directional,the percentage of phase related information increases off axis. Forimproved focus for the phantom stereo image, placement of the speakers,like pictures, at the usual standing person height, confers the benefitof a moderate off-axis placement for the normally seated listeneroptimising the stereo effect. Likewise the triangular left/rightgeometry with respect to the listener provides a further angularcomponent. Good stereo is thus obtainable.

There is a further advantage for a group of listeners compared withconventional speaker reproduction. The intrinsically dispersed nature ofacoustic panel sound radiation gives it a sound volume which does notobey the inverse square law for distance for an equivalent point source.Because the intensity fall-off with distance is much less than predictedby inverse square law then consequently for off-centre and poorly placedlisteners the intensity field for the panel speaker promotes a superiorstereo effect compared to conventional speakers. This is because theoff-centre placed listener does not suffer the doubled problem due toproximity to the nearer speaker; firstly the excessive increase inloudness from the nearer speaker, and then the corresponding decrease inloudness from the further loudspeaker.

There is also the advantage of a flat, lightweight panel-form speaker,visually attractive, of good sound shown in our co-pending applicationSer. Nos. 09/011,773, 09/011,770, and 09/011,831; and

The transducers (70,71) are driven by an amplifier (10) coupled inparallel to the respective transducers with the interposition of astep-up transformer (72) and matching resistance (73) in the line to thepiezo transducer in view of its relatively high voltage requirement. Ifdesired more than one transducer (70) and/or transducer (71) may beprovided to improve the performance.

FIG. 5 illustrates how a distributed mode panel (2) according to thepresent invention, e.g. of the kind shown in FIGS. 1 and 2, can bedriven by a pair (70,74) of transducers (9), the transducer (70) being ahigh frequency piezo-electric transducer e.g. of the kind shown in FIG.7 and the transducer (74) being a low frequency piezo-electrictransducer of the kind shown in our co-pending application Ser. No.09/011,770. Reference (75) indicates that the transducer (74) isweighted with a mass to increase its inertia. The transducers (70,74)are driven by an amplifier (10) to which they are connected in parallel,with resistors (78) interposed to provide a frequency dividing network.If desired more than one transducer (70) and/or transducer (74) may beprovided to improve the performance.

FIG. 6 illustrates how a distributed mode panel (2) according to thepresent invention, e.g. of the kind shown in FIGS. 1 and 2, can bedriven by a pair (68,69) of electrodynamic transducers, e.g. of thekinds shown in our co-pending application Ser. Nos. 09/011,773,09/011,770, and 09/011,831.

The transducer (68) is intended as a high frequency driver and is thusof low inductance, whereas the transducer (69) is intended as a lowfrequency driver and is of high inductance.

The transducers (68,69) are driven in parallel by an amplifier (10) witha capacitor (77) in the line to the transducer (68) to act as afrequency divider to pass most of the high frequency signal to thetransducer (68). If desired more than one transducer (68) and/ortransducer (69) may be provided to improve the performance.

FIG. 7 shows a transducer (9) for a distributed mode panel (2) in theform of a crystalline disc-like piezo bender (27) mounted on a disc(118), e.g. of brass, which is bonded to a face of the panel (2), e.g.by an adhesive bond (20). In operation an acoustic signal applied to thetransducer (9) via leads (28) will cause the piezo disc (27) to bend andthus locally resiliently deform the panel (2) to launch bending wavesinto the panel.

What is claimed is:
 1. A panel-form loudspeaker (81) comprising:a firstmember (2); a second member (4); a resilient suspension (3) couplingsaid first and second members; each of said members (2,4) comprising astiff lightweight panel having capability to sustain and propagate inputvibrational energy by bending waves in at least one operative areaextending transversely of thickness to have resonant mode vibrationcomponents distributed over said at least one area and havepredetermined preferential locations or sites within said area fortransducers (9); a first transducer (9) mounted on said first member (2)at one of said locations or sites to vibrate said first member to causeit to resonate forming an acoustic resonator which provides an acousticoutput when resonating; and a second transducer (9) mounted on saidsecond member (4) at one of said locations or sites to vibrate saidsecond member to cause it to resonate forming an acoustic resonatorwhich provides an acoustic output when resonating.
 2. A panel-formloudspeaker according to claim 1, wherein the second member (4) ismounted in an aperture (82) in the first member (2).
 3. A panel-formloudspeaker according to claim 1, wherein the second transducer ismounted wholly and exclusively on the second member (4).
 4. A panel-formloudspeaker according to claim 2, wherein said resilient suspension (3)is attached to the edge of said second member (4) and the edge of theaperture (82) in said first member (2).
 5. A panel-form loudspeakeraccording to claim 4, wherein said transducers (9) are mounted whollyand exclusively on their respective members (2, 4).
 6. A panel-formloudspeaker according to claim 5, wherein said first and secondtransducers (9) are adapted to operate in different frequency ranges.